Wireless communications capabilities are increasingly being integrated into portable devices, including laptop computers, handheld devices (such as personal digital assistants (PDAs)), and mobile phones. The integration of such capabilities can provide users with anywhere and anytime connectivity to information resources.
Many communications systems include multiple base stations dispersed across regions to provide mobile access to network resources. One such system is WiMAX, as provided by the Institute of Electrical and Electronic Engineers (IEEE) 802.16 standards. In WiMAX networks, mobile stations may exchange signals with base stations. These signals may be modulated according to orthogonal frequency division multiplexing (OFDM) techniques.
Communications networks may include different types of base stations (BSs). For example, WiMAX networks provide for macro BSs and femto BSs. Macro base stations offer relatively large coverage areas (called cells) that are suitable for extensive outdoor coverage (and some penetrating indoor coverage). In contrast, femto base stations, which provide small coverage areas (called femto cells) are typically suited for indoor areas like homes and offices.
For networks employing OFDM, synchronization among neighboring BSs is critical. This is because timing offsets between such BSs will incur severe interference. For typical deployments, such as ones employing urban macro cells, synchronization can be maintained with technologies, such as the global positioning system (GPS), that provide accurate timing references.
However, it is likely that indoor femto BSs will be unable to receive GPS signals. Unfortunately, these femto BSs may still have overlay radio coverage with neighboring macro-BSs. Thus, it is desirable to achieve good synchronization between femto BSs and macro BSs.